The present invention relates to the production of threaded screw elements including elements having shafts of small diameter or large diameter, solid or hollow, metallic or plastic, rigid or flexible, and to nut assemblies thereof such as drive assemblies for converting rotary motion of the screw element or of the nut member to relative linear displacement. Such drive assemblies are useful for the rapid, smooth and precise adjustment of linear mechanisms, such as zoom lenses, by automatic screw-turning or nut-turning means.
Conventional lead screws are formed by machine-cutting threads into the surface of a rigid metal shaft. In order for the threads to be resistant to frictional wear during use, resulting in loss of precision, the metal shaft must be solid and rigid and formed of hardenable metal, the screw must be hardened after the thread is formed, and the thread grooves must be ground in order to produce a sufficiently accurate lead surface. These operations are tedious and time-consuming, require the use of a metal shaft having a diameter which is not too large or too small for machine-cutting, and result in rigid lead screws which are not as wear-resistant or smooth-operating as desired, particularly in the absence of lubrication. Conventional lead screws are assembled with recirculating ball nuts and are limited to the thread pitch being relatively coarse in order to accommodate the recirculating steel balls, which coarse thread does not permit direct adjustment drive by means of a small D.C. motor. Such automatic drive is desirable for applications where rapid adjustment is necessary.
Also, conventional lead screws have the entire shaft formed of the same hardenable metal required for the threaded surface. Since the bulk of the lead screw is present in areas other than the motion-imparting thread area, it is desirable to be able to form such other areas of the screw from materials which are less expensive and easier to form, such as of molded plastics or softer metals.